Generally, an electroluminescent device (such as a display) comprises a plurality of devices emitting visible radiation, best known as pixels, which are arranged such as to form a matrix of light spots. Usually, for each of the emitting devices, the light emission occurs at a preset wavelength, corresponding to the wavelength of one of the three basic colors: red R, green G, and blue B (“RGB”).
In an electroluminescent device, the individual pixels are grouped in macro-pixels comprising a first pixel for emitting the red light, a second pixel for the green light and a third pixel for the blue light. Thereby, each macro-pixel is capable of emitting light in any color as a combination of the three basic colors RGB.
Currently marketed displays are liquid crystal displays (LCD) or displays employing silicon emitter devices, i.e. junction diodes.
In recent years, a considerable research activity has been addressed to the development of electroluminescent devices manufactured using materials alternative to silicon, such as organic polymer materials.
In an electroluminescent device manufactured using organic polymer materials, each of the visible radiation emitter devices is a multilayer structure consisting of an anode and a cathode of a conductive material, one or more layers of organic polymer material being interposed therebetween. These devices are known as Organic Light Emitting Diodes (OLEDs). As conventionally established in this field, the anode is a hole injector electrode whereas the cathode is an electron injector electrode.
The external behavior of an OLED is similar to that of a visible radiation emitter device made of silicon (i.e. a p-n junction) in which the visible radiation is generated by applying a potential difference between anode and cathode. In fact, also in the case of an OLED, by polarizing the device, i.e. applying a potential difference between anode and cathode, mechanisms are generated within the layer of polymer material that cause the emission of light radiation.
Particularly, a first OLED is known in which the layer of polymer material comprises at least two different layers having the function of Hole Transport Layer (HTL) and Electron Transport Layer (ETL), respectively. Qualitatively, in this type of OLED, by polarizing the emitter device, holes are injected from the anode and electrons from the cathode that can reach the layers HTL and ETL, respectively, thereby forming electron-hole pairs (also known in the literature as the exciton). Each of these electron-hole pairs is in the so-called excited state, and as it is understood by those skilled in the art, it is subjected to a decay step to pass from the excited state to the ground state. It is during this quantum transition that the emission of light radiation is generated.
Furthermore, a second type of OLED is known in which the layer of polymer material is made by an individual layer having the double function of Hole Transport Layer (HTL) and Electron Transport Layer (ETL). The operating principle of this type of OLED is similar to those qualitatively described above with reference to the first typology.
The organic polymer materials used for the first type of OLEDs are low molecular weight polymer materials (clusters of a few molecules, oligomers) whereas those of the second type of OLEDs are high molecular weight polymer materials (clusters of many molecules, polymers).
The use of organic polymer materials highly influences the selection of the layer deposition techniques and the definition of the geometries to be employed in the process for manufacturing an electroluminescent device. In fact, the organic polymer materials (both of high molecular weight and low molecular weight) are very sensitive to those deposition techniques and definition that are normally used for layers of inorganic materials. In fact, the conventional photolithography techniques cannot be used on organic polymer materials since they are very delicate materials which are unlikely to withstand etching techniques. The evaporation deposition technique and definition through shadow mask is successfully used for defining precise geometries in the polymer materials.
The electroluminescent devices made of organic polymer materials of the known type have drawbacks resulting from electrical and optical losses causing the undesired effects of electric crosstalk and optical crosstalk, respectively, between OLEDS arranged proximate to each other.
In fact, it has been noted that, by using conventional deposition techniques (shadow mask evaporation) the polymer materials are diffused not only vertically but also laterally. This entails that two layers of organic material belonging to OLEDs, which are different but placed side by side, can be in contact with each other. Since the polymer materials are also electrical conductors, a spurious electric path is created in which part of a OLED control electric current is carried to an OLED that is adjacent thereto, thereby causing the same to be powered, by electric crosstalk, despite it is not polarized.
These types of losses cause a low luminosity of the electroluminescent device since part of the electric current intended for supplying an individual pixel is lost. Typically, this drawback is limited by supplying the device with a greater power, but this inevitably implies a greater supply power consumption than in the ideal operation.
The optical crosstalk is exhibited, for example, when an OLED adjacent to a powered one turns on. This is due to part of the light radiation emitted by the powered OLED that can be lost in a different direction from the main direction of radiation, such as side emissions. The photons of side emissions reach the adjacent OLED and cause the same to turn on, by photoluminescent effect.
Furthermore, side emissions can, in some ways, be channeled in spurious optical paths that are defined between those spaces separating the plurality of OLEDs in a display, thereby interference between the OLEDs because of optical crosstalk is more likely to occur.
When the pixels are partially turned on and others are undesirably turned on, or unwillingly turned off, the image definition is very poor.
To overcome the drawbacks set forth above, which have been found to be mainly due to a too close distance between adjacent pixels, attempts have been made to increase this distance but this entails a reduction in the electroluminescent device resolution.
There is a need for an electroluminescent device made of organic material which allows reducing the crosstalk, and particularly, restraining the electrical crosstalk between adjacent pixels.